Many wind turbines comprise a rotor with its axis in a horizontal position. Wind turbines with horizontal axis require yawing. The yaw angle error is the angle between the rotor axis and the wind direction. To achieve maximum capacity of the wind turbine the rotor axis should be parallel to the wind direction. This means that the yaw angle error has a value of 0°. In this consideration, a vertical tilt angle many rotor axes have in order to prevent the blades from touching the tower at high wind speeds and which, strictly speaking, means that the rotor axis is actually slightly off the parallel orientation, is neglected.
During the yawing process the turbine nacelle is turned around a vertical axis until the rotor axis is, except for a possible vertical tilt angle, parallel to the wind direction. Usually the yaw axis is concentric with the wind turbine tower axis. Yawing is normally carried out by electrical or hydraulic means. The yaw drive unit control is based on a measurement of the wind direction by one or more wind direction sensors placed on the turbine nacelle.
When the wind direction sensors are located on the turbine nacelle of the wind turbine with a rotor placed upwind of the tower, the wind direction sensors are not exposed to free, undisturbed wind. Instead, the wind has passed through the rotor and may be distorted by this rotor passage and by speed-up phenomena around the nacelle itself. Such a distortion is usually a function of wind speed, turbulence, wind direction and vertical wind inclination. Consequently, the yaw alignment of the rotor axis with the wind direction is associated with some uncertainty when carrying out yaw control based on nacelle-mounted wind direction sensors.
At low and medium wind speeds the power output is sensitive to proper yaw alignment. It is generally believed that on wind turbines the relationship between yaw alignment and power output is a cosine-square function, rather than a simple cosine function, as would be expected. The reason for this high sensitivity is related to the downwind wake behaviour of the wind turbine.
If the cosine-square relationship is valid, a 5° yaw angle error corresponds to a power output of cos2(5°)=0.99, which means a 1% loss of power. While a loss of this magnitude may sound marginal, it easily runs into more than 100,000 kWh annually for a large wind turbine. Furthermore, yaw angle errors create higher dynamic loading on the wind turbine structure which is an unwanted phenomenon.